In the fields of biology and medicine, there is a high degree of interest in sensors for selective, marker-free, high-sensitivity analysis of very small fluid quantities. One possibility for constructing such sensors is based on the use of optical cavities as microresonators, in particular in the form of toroids, goblets, discs, ellipsoids or spheres. At particular wavelengths λ, resonances form in the cavity. If molecules of an analyte become attached to the resonator surface, the effective radius R of the cavity increases due to a change in the refractive index n in the environment of the microresonator. A change in the radius R and the refractive index n brings about a change in the resonance wavelengths λr, as given by:
            Δλ      r              λ      r        =                    Δ        ⁢                                  ⁢        R            R        +                            Δ          ⁢                                          ⁢          n                n            .      
According to F. Vollmer and S. Arnold in Whispering-gallery-mode biosensing: label-free detection down to single molecules, Nature Methods 5 (2008) 591-596, by analysing the spectrum, it is possible, from a shift in the wavelength of the resonance, to deduce attachment of molecules.
In order to be able to detect very small molecule quantities, microresonators with high quality factors are required. A. M. Armani, R. P. Kulkarni, S. E. Fraser, R. C. Flagan and K. J. Vahala describe in Label-Free, Single-Molecule Detection with Optical Microcavities, Science 317, pp. 783-86, 2007, label-free detection of individual molecules applied to the surface of a microresonator. A toroid made of silicon dioxide and mounted on a silicon foot on a silicon substrate was used as a microresonator.
T. Grossmann, M. Hauser, T. Beck, C. Gohn-Kreuz, M. Karl, H. Kalt, C. Vannahme, and T. Mappes describe in High-Q conical polymeric microcavities, Appl. Phys. Lett. 96 (2010) 013303, a method for producing microgoblet resonators made of polymethyl methacrylate (PMMA), which is distinguished by having a high degree of transparency in the visible spectral range, having a quality factor of above 106.
In order to couple light into the cavity, evanescent coupling is used. J. Knight, G. Cheung, F. Jacques and T. Birks describe in Phase-matched excitation of whispering-gallery-mode resonances by a fiber taper, Opt. Lett. 22 (1997) 1129-1131, coupling into a microresonator by means of adiabatically thinned glass fibres. In order to obtain the most effective possible coupling, the diameter of the glass fibre must be thinned to values in the range of 0.1 μm to 3 μm. Due to the small diameter, the glass fibre becomes very fragile and handling thereof is made difficult. For the coupling between the fibre and the cavity, the distance must be set to values less than the wavelength of the irradiated light. This adjustment requires a high degree of positional accuracy and is possible only with micrometer tables and under controlled laboratory conditions. If the microresonator is used in a sensor for analysing a fluid, the adjustment is made more difficult by flow within the analyte.
EP 2287592 A1 discloses a micro-optical component for coupling laser light to microresonators, comprising at least one waveguide for laser light and at least two microresonators, each having the form of a rotationally symmetrical body arranged on a foot, preferably designed as a spheroid or a toroid, wherein the at least two microresonators are mounted on a first substrate which is provided with first side walls and the at least one waveguide is mounted on a second substrate which is provided with second side walls, such that the first side walls and the second side walls are rigidly connected to one another.
Resonance frequencies of the cavity create characteristic gaps in the transmitted spectrum in the waveguide, which are known as Lorentz curves. In order to resolve fine displacements of these resonances when molecules become attached to the structure of the resonator, the excitation must be carried out with a continuously tunable laser. The excitation frequency must follow the displacement of the resonance frequency. For this purpose, the whole spectral range being investigated is often continuously scanned with the excitation laser. The spectral analysis of the transmitted light must be carried out with a high resolution in order to detect the finest displacements, and for this purpose a spectrometer or a photodiode which is read out synchronously with the excitation laser is required.
In order to avoid complex adjustment of the glass fibre or waveguide, microresonators are coated or doped with an optical amplifier material, in particular a dye. If a doped cavity is pumped with an external laser having a dye-specific wavelength, a coherent emission can be stimulated. The spectrum emitted by the microresonator is characteristic of the geometry of the cavity and the active material being used. Due to the attachment of molecules from the analyte onto the resonator surface, apart from the resonance frequency, the emitted spectrum of the active microresonator is also displaced. This displacement serves as a sensor signal.
Microresonators doped with an active material emit light isotropically in the “resonator plane” along the whole periphery. Typically, the light emitted is collected with the end of a glass fibre or a lens. Due to the small aperture of the glass fibre, however, only a small part of the emitted light can be collected and detected. Since only a small part of the emitted light is scattered out of the plane at surface defects of the microresonator, it is also only a small intensity that can be collected with a lens positioned over the substrate. Using the above-mentioned detection methods, only a low signal-to-noise ratio is achievable.
In order to obtain directional emission, M. Kneissl, M. Teepe, N. Miyashita, N. M. Johnson, G. D. Chern and R. K. Chang describe in Current-injection spiral-shaped microcavity disk laser diodes with unidirectional emission, Appl. Phys. Lett. 84 (2004) 2485, a spiral-shaped resonator geometry which, in place of isotropic emission, enables directional emission. Due to the modified form of the microresonator, although the signal-to-noise ratio increases, the quality factor of the microresonator falls markedly, such that low molecule concentrations cannot be detected with this apparatus.
U.S. Pat. No. 7,387,892 B2 discloses a biosensor which is based on active rotationally symmetrical microresonators made from GaN/AlGaN. The emitted light is read out with integrated rows of photodiodes. As U.S. Pat. No. 7,310,153 B2 discloses, mounted on the row of photodiodes is a wedge-shaped thin-film filter which ensures that only particular wavelengths impinge on individual diode fields. Due to a resonance shift when an attachment from the analyte takes place, the intensity distribution on the photodiodes is altered. However, light emitted from the microresonator impinges on the detector from only a small angular segment, such that only a low signal-to-noise ratio is obtained. Due to the large distance between the microresonator and the detector of up to 1 cm, only a few microresonators can be placed on a substrate. Furthermore, a separate detector is provided for each microresonator, such that the complexity of the construction and connection technology is increased, since the detectors are manufactured on a separate substrate and are only subsequently mounted on the resonator substrate. Since the accuracy of detection depends on the number of photodiodes in a row, as the accuracy increases, the number of connections for reading out from the photodiode row also increases.